This invention relates to the field of organic chemistry and more particularly to the preparation of pure 2-ethylhexyl gallate.
Various alkyl esters of gallic acid have been used as antioxidants/stabilizers in a number of food, drug and industrial chemical formulations. Such esters have also found application as components of chromogenic combinations in pressure-sensitive copying compositions. Such esters may be prepared by the reaction of gallic acid with the appropriate alcohol. Gallic acid is obtained by the hydrolytic degradation of naturally occuring gallotannins, coupled with appropriate purification procedures.
The esterification of gallic acid with an alcohol normally yields a crude ester product which contains a variety of impurities, some colored, carried over as impurites in the gallic acid and/or as a result of side reactions associated with the esterification. Most applications of such esters require that the ester be relatively free of impurities, particularly of colored bodies.
In the production of the 2-ethylhexyl ester of gallic acid, it has been common practice to crystallize the crude esterification product from a xylene solution. Such treatment produces very erratic results. On rare occasions it yields a readily filtrable product which is essentially colorless and free flowing. However, in most instances, this treatment produces a viscous slurry that is very difficult to filter. The use of benzene and toluene gives results similar to those obtained with xylene. Several other types of solvents are also unsatisfactory for varying reasons.
The use of certain aliphatic solvents solves the filtration problem, but leads to another. Crude 2-ethylhexyl gallate is difficulty soluble in hot heptane, from which, on cooling, it crystallizes in readily filtrable form. However, the resulting crystals retain the colored impurities present in the crude product. Hexane gives similar results. Thus it is seen that there is a need for a solvent system which enables one to crystallize 2ethylhexyl gallate in the form of readily filtrable crystals which are essentially free of colored impurities.